Clean up StudentCourseJoinAuthenticatedAction:
1. simple stuff to improve readability
2. changed to lower level methods to increase testing speed
3.spotted unreachable exception handling case, added a TODO for future logic and AccountLogic exception issue for method joinCourseForStudent
minor typoes fixed
//TODO simplified for unreachable branch
comma moved cosmetic reason
Merge branch 'master' into Issue1959
[1959] Clean up StudentCourseJoinAuthenticatedAction

fixes #1959